Help - Reverse draft from water heater CO monitor going off

I have a 15 year old home with a regular venting hot water heater. It is a 3" B vent to the roof that goes through the wall upstairs wall, through the attic above the garage and out the top of the roof.

The water heater is 3 years old and we have never had a problem before. In late October we had our house reroofed due to a hail storm. Problems with CO began to occur off and on around Thanksgiving. It seems if temperature is below 10 deg F. We get a reverse draft in the vent causing CO to leak into the house. You can also smell the combustion gases.

The water heater vent had a 24" striaght up run then a 90 another 24 inch run then another 90 to the pipe going straight up to the roof. I changed this this week to go to 2 45's to straighten the run and always keep an upslope. This helped until last night after both kids baths and a cold clear night I had the CO monitor going off. I cracked a window in the basement and kept fans going to lower the level.

Because of snow/ice I can not really get on the 9/12 pitch roof safely and check the outlet cap. I am also concern I may have the house too tight. I recently installed a new patio door because the other one had several leaks around it and we went from traditional box attic vents to ridge cap. Could this have tightened the house that much?

Should I invest in a power vent water heater when I have a perfectly good one. Should I invest in a power vent kit for my hot water heater? Should I put in a make up air vent - Not exactly sure how these work, where to get them. Any help would be greatly appreciated.

Might they have crimped the pipe where it goes through the roof? Do you have a big snow accumulation on the roof? Generally, unless it is designed for it, you can't retrofit a power vent. The ridge cap vs box vents shouldn't make a difference although if your house does leak air into the attic, that would likely exhaust it better. If this happens regularly, you could try leaving a basement window cracked to provide makeup air as an experiment. Most houses are loose enough where that isn't a big factor unless you have a combination of events: the furnace is running, the dryer is running, the range hood is exhausting, and maybe bathrooms. All of these things are trying to exhaust air and it has to come in somehow - the flue for the WH might be the easiest place. As we tighten up homes, providing makeup air for these types of things becomes more important to engineer into the house. Many of the newer and more efficent units are closed combustion, they draw their own combustion air from outside. A power vent doesn't, it is typically used where you need to force the air further or out a different path than a conventional flue would support. An air-to-air heat exchanger has the ability to help balance the load and recover most of the energy you've paid for, but those that will work in super cold climates get much more challenging and expensive.

I have a 15 year old home with a regular venting hot water heater. It is a 3" B vent to the roof that goes through the wall upstairs wall, through the attic above the garage and out the top of the roof.

The water heater is 3 years old and we have never had a problem before. In late October we had our house reroofed due to a hail storm. Problems with CO began to occur off and on around Thanksgiving. It seems if temperature is below 10 deg F. We get a reverse draft in the vent causing CO to leak into the house. You can also smell the combustion gases.

The water heater vent had a 24" striaght up run then a 90 another 24 inch run then another 90 to the pipe going straight up to the roof. I changed this this week to go to 2 45's to straighten the run and always keep an upslope. This helped until last night after both kids baths and a cold clear night I had the CO monitor going off. I cracked a window in the basement and kept fans going to lower the level.

Because of snow/ice I can not really get on the 9/12 pitch roof safely and check the outlet cap. I am also concern I may have the house too tight. I recently installed a new patio door because the other one had several leaks around it and we went from traditional box attic vents to ridge cap. Could this have tightened the house that much?

Should I invest in a power vent water heater when I have a perfectly good one. Should I invest in a power vent kit for my hot water heater? Should I put in a make up air vent - Not exactly sure how these work, where to get them. Any help would be greatly appreciated.

Thanks

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First, with a properly burning gas burner the CO level should be EXREMELY low- low enough that the monitor shouldn' t be going off. Take a peek at the flame- you may need to clean up the burner if you see a lot of yellow in it.

As long as the unit isn't sharing a flue it should have sufficient draft to not backdraft in most reasonbly (but not super) tight houses. You should be able to test for backdrafting (even when it's not firing) by turning on the clothes dryer and all kitchen & bathroom exhaust fans, and if your heating if forced hot air run the air handler (burner on/burner off) as well. Using a thread or spider web at the water heater's draft hood to see if it's backdrafting. Sequentially turn off them off to see if the flow changes to the correct direction. If it's backdrafting when NOTHING is de-pressurizing the house, (rare) you probably have some other flue that's drafting harder.

If the top of the stack isn't at least a few feet above the highest ridge of the house wind currents over the roof can intermittently create backdrafting pressures. Lengthening the stack and adding a (stainless steel, sutiable for natural gas exhuast) venturi-type terminator may be enough to correct those conditions if it's just on the edge, working-mostly. Insulating the portion in the uncontitioned garage attic may help too, but only in marginal cases.

Any opening from the exterior into the basement will neutralize some of whatever is depressurizing the house and will help maintain the correct stack flow. How big it needs to be depends on how much depressurization is coming from other sources.

If retrofitting a bunch of these remedies costs more than sealed-combustion water heater, you know what to do. Sealed combusion direct-vented is the only way right way forward in any heating/water-heating appliance in my book. Building/retrofitting a house to be as tight as possible is "right" for any number of reasons. Relying on random leakage for ventilation air is a recipe for issues like mold & rot in any cold-climate home with decent R-values, not just backdrafting of old-skool burners. If-your heating system is hydronic (pumped hot water), going with an indirect-fired hot water tank operated as a zone off the heating system boiler is reliable & efficient, and may be roughly same money as a direct-vented or power-vented standalone installation.

How many possibilities do you want.
1. The house could be "tight" with something exhausting air so the only place for "replacement air" is down the flue/
2. The temperature could be so cold that the hot gases are cooling off and thus losing their convection currents which means they "fall" back down the flue.
3. The flue could be blocked.
These are the most common causes, but YOU could have some other, esoteric, situation which is causing it.

Well tonight it is just as cold, but I left the basement window cracked all day. the vent pipe was coild, but not ice cold like yesterday. I started the hot water heater manually (turned it all the way up) and walla the flue got instantly hot. I didn't leave it run long enough to build up the CO. I'll wait until tomorrow morning when showers are going and see how it does. If that works I'll put in a fresh air vent this weekend. If not back to several of the other items you guys have mentioned. Thank you very much for all the help.

I do not have a particularly tight house, but I cannot use the hood when the Masonry fireplace is burning [ and I have make up air built into the fireplace]. Slam a door and another exterior door shakes.

I would do the vent with a very sensitive damper to keep most of the cold air out.

Tightening your house (i.e. replacing patio door), may have caused other air exhausting appliances to depressurize your home. The following should be checked, bathroom and kitchen fans, dryer, and forced air furnace. With water heater off, turn on all bath/kitchen exhaust fans and dryer. ALL at once. Place burning match/burning incense near water heater hood and see if smoke is drawn up flue. if no draw, try turning off fans until you get draw.

A second possibility that I had was leaking return air ducts in an unfinished basement. Large gaps in the return air ducts were causing furnace to suck air from basement. There were no supply ducts as it was unfinished. When door from 1st floor to basement was closed, no source of air, so flue back drafted into basement. Soot and debris on top of water heat and plastic bushings around hot and cold line had partially melted. However, the nearby CO detector never alarmed, but did have a non-zero reading. Note that this was not a continual backdraft and only occurred if the furnace blower was running.